Spring Validation Concepts

External

• https://docs.spring.io/spring/docs/5.1.0.RELEASE/spring-framework-reference/core.html#validation
• https://docs.spring.io/spring/docs/5.1.0.RELEASE/spring-framework-reference/core.html#validation-beanvalidation
• https://spring.io/guides/gs/validating-form-input/

Internal

• Spring Framework Concepts
• Bean Validation

Overview

Spring supports Bean Validation API (JSR-303) which allows to declare validation rules as annotations, as opposed to explicitly writing validation logic in the application code. The Bean Validation API and the Hibernate implementation of the Validation API are automatically added to the project as transient dependencies of the Spring Boot web starter.

Bean Validation in Spring MVC

Applying bean validation in Spring MVC consists of:

Declare Validation Rules

Declare validation rules on the domain model data class that is to be validated. Annotate members of the domain model data classes with Bean Validation annotations @NotNull, @Size, @NotBlank, @Pattern, @Digits or Hibernate Validation annotations @CreditCardNumber

Configure Controller Methods for which Validation Should Be Performed

Designate controller methods for which validation should be performed by annotating the domain model data objects that forms bind to with @Valid. The validation is performed after the domain model object is bound to the submitted form data and before the controller's handler method is called. Spring Validation provides an org.springframework.validation.Errors class that can be added to the controller method signature and whose instance carries validation error information inside the method, if validation fails. If there are any validation errors, the details of those errors will be captured in the Errors object, and the handler method should conclude without processing the data and should return to the view that generate the faulty data, so the form can be redisplayed, together with corresponding error messages. Note that the form should be explicitly configured to display the error messages, if any.

@Controller
@RequestMapping(...)
public class SomeController {

    @PostMapping
    public String create(@Valid Taco taco, Errors errors) {

        if (errors.hasErrors()) {

            // return the logical name of the view that 
            // renders the form that produced the errors

            return "someView"; 
        }

         // no errors, process the form data ...
    }
}

Configure Forms to Display Validation Errors

Configuring the form views to display validation errors. Thymeleaf offers access to the Errors object via the "fields" property and with its "th:errors" attribute.

<span class="validationError"
       th:if="${#fields.hasErrors('ccNumber')}"
       th:errors="*{ccNumber}">CC Num Error</span>